In the well-known process of xerography, or electrophotographic printing, a charge retentive surface, known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as "toner." Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In the process of electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as "development." The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with the toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. The brush is brought into contact with the imagewise-charged photoreceptor, and the toner particles move from the chains of carrier beads to adhere to the photoreceptor.
Another known development technique involves a single-component developer material. In a typical single-component development system, each toner particle has both magnetic properties (to allow the particles to be magnetically conveyed to the photoreceptor) and an electrostatic charge (to enable the particles to adhere to the photoreceptor). In such a system, the developer roll is in the form of a cylindrical sleeve which rotates about a stationary magnet assembly. Thus, magnetized toner particles adhere to the rotating sleeve by the force of the stationary magnets within the sleeve, and as the sleeve rotates around the magnets, particles adhering to the sleeve will be exposed to an alternating series of magnetic polarities. A metering blade is typically in continuous contact with the toner particles on the sleeve along one longitude of the developer roll, so that the toner particles will adhere to the moving sleeve in a thin, uniform coating. When this thin layer of particles is obtained, the developer roll advances the toner particles to a development zone adjacent the surface of the photoreceptor. In the development zone, the toner particles adhering magnetically to the developer roll are attracted electrostatically to the latent image recorded on the photoreceptor. With this technique, toner particles may be evenly distributed on the charged areas of the latent image. Systems in which the toner particles are caused to adhere to the devloper roll by other than magnetic forces, such as electroststaic forces, are also known.
An important variation to the general principle of single-component development is the concept of "scavengeless" development. The purpose and function of scavengeless development is described more fully in, for example, U.S. Pat. No. 4,868,600 or U.S. Pat. No. 4,984,019. In a scavengeless development system, toner is conveyed to the photoreceptor by means of AC electric fields supplied by self-spaced electrode structures positioned within the nip between a donor roll and photoreceptor. The electrode structure, commonly in the form of wires extending adjacent to and across the photoreceptor, is placed in close proximity to the donor roll within the gap between the donor and the photoreceptor. Generally, this type of development is useful for devices in which different types of toner are supplied onto the same photoreceptor, as in tri-level or "highlight" xerography.
A typical scavengeless development apparatus includes, within a developer housing, a donor roll disposed adjacent the photoreceptor. In the nip between the donor roll and the photoreceptor are wires forming the electrode structure. During development of the latent image of the photoreceptor, the electrode wires are electrically biased relative to the donor roll, to detach toner therefrom to form a toner powder cloud in the gap between the donor roll and the photoreceptor. The charged areas on the photoreceptor corresponding to the latent image electrostatically attract toner particles from the powder cloud, forming a toner powder image thereon.
For both scavengeless and regular types of single-component development, another proposed apparatus for placing an even layer of electrostatically charged toner particles on the surface of a developer roll is a charge rod. The general principle of the charge rod is described in U.S. Pat. No. 4,459,009, as well as in U.S. Pat. No. 4,505,573 and U.S. Pat. No. 4,876,575. In essentials, an electrically biased charge rod is used in place of the metering blade against the surface of the developer roll, and serves not only to regulate the thickness of the layer of toner particles on the developer roll, but also to impart an electrostatic charge to the toner particles. Typically, the charge rod rotates in a direction against that of the surface of the developer roll.
In any kind of developing system, one of the most important considerations is the electrostatic charge per unit mass of the toner particles, known as "q/m." The specific value of q/m under given conditions has a substantial effect on the ultimate image quality of prints made with the apparatus. Generally speaking, the q/m for toner particles in a development process must be within a certain range for acceptable print quality. If the q/m is above this range, the toner particles will require more force to "jump" from the developer roll to the photoreceptor in the developing step, and in the transfer step, more force will be required to cause the particles to transfer from the photoreceptor to paper. Alternatively, a too low q/m will increase the likelihood that toner particles will adhere even to relatively discharged areas on the photoreceptor and appear in background areas of finished prints, causing a "dirty" appearance to the print. In a two-component developer system, the q/m of the toner may be relatively easily controlled, to a degree, by controlling the ratio of toner particles to carrier particles in the developer mixture. However, this straightforward technique is not possible with a single-component developer system.
In the prior art there are many proposed techniques for determining the q/m of toner particles in the course of the xerographic process. U.S. Pat. No. 4,026,643 discloses and apparatus and method wherein q/m is determined by combining a measurement of the difference between the photoreceptor potential in the presence and in the absence of charged toner particles with a measurement of a difference in optical reflectance in the presence and in the absence of charged toner particles. The measurement of the difference in the electrostatic potential of the photoreceptor provides a quantity proportional to the toner particle charged per unit area. The measurement of the difference in optical reflectance provides a quantity related to the toner mass per unit area, a quantity that is linear for low particle densities. Combining the two difference measurements provides a value proportional to the q/m of the toner.
U.S. Pat. No. 5,005,050 discloses an apparatus for controlling q/m. A pair of electrometer probes measure the potential of the photoreceptor before and after development. The output of the electrometers are compared at a difference network, and the difference between the signals is proportional to the amount of charge deposited on the belt due to movement of toner particles from the development station to the photoreceptor surface. The q/m is then adjusted by means of ions directed to the toner particles. U.S. Pat. No. 5,047,802 describes a variation of this technique.
U.S. Pat. No. 5,006,897 discloses a piezo device positioned to interact with toner particles. The device is periodically electrically biased to attract toner particles, and the frequency shift of the piezo device due to the mass of attracted toner particles is detected. By appropriately measuring the electrical characteristics of a capacitive circuit, the charge of toner particles on the developer mixture positioned between two capacitive plates can be determined. In this way, the q/m of toner particles may be measured in the course of a xerographic process.
U.S. Pat. No. 5,040,021 discloses a densitometer for determining the optical density of toner on a photoconductive layer by exposing the photoreceptor to radiation through the toner layer such that the amount of exposure is characteristic of the density of the toner layer. The change of voltage on the surface of the toner layer caused by the exposures detected such that the change in detected voltage is characteristic of the density of the toner layer. The measurement of the change in the detected voltage is carried out by measuring the characteristic voltage on a toner layer, discharging the characteristic voltage on the photoreceptor, and then measuring the remaining voltage.
JP-A-3-276174 discloses a system for controlling the quantity of toner material on a "replenisher roll." A photosensor detectes the desnity of toner material on the surface of the roll, and in response thereto, the position of a metering blade is adjusted to vary the quantity of toner on the roll. In this reference no suggestion is made of a relationship between the mere density of toner on the surface and the charge per unit mass of the toner.